Telerobotics involves the manipulation and control of objects and devices over a long distance. The user controls a remote device by means of control signals transmitted over a particular medium. In order to enhance the user's ability to "feel" the effects of his or her control inputs, force reflection is often included in which the remote device feeds force or other motion relative signals back to the user so that control adjustments may be made based upon the remote device's state. Such telerobotic system have been employed variously in underwater applications by means of control signals carried by sound waves, in underground environments and have been contemplated for use in long range space applications utilizing radio waves as well as in microscopic environments such as remote microsurgical instruments. A generalized diagram of such a system is illustrated in FIG. 1.
As long as time delays in the transmission or communication 20 of control signals to the remote telerobotic device or "slave" 22 from the "master" system 24 and from the remote device 22 back to a master system 24 (controlled by the user 26) remain relatively small, the actions of the user 26 and remote device's effects on the environment 28 are relatively synchronized. Thus the system, while still retaining a tendency toward instability, remains controllable in most instances. A human user 26 generally exhibits a quantifiable reaction time (approximately 1/10 of a second) and, thus, as long as time delays remain in this range, the system is transparent, meaning that the user is unaware of any time delay. As such, a control movement that is quickly followed by a remote device force response at the master system 24 or other motion state feedback to the user does not effect the user's ability to operate the system properly.
However, when dealing with particularly long operating ranges such as in space, or when the transmission medium is slower than the speed of light such as for sound waves, time delays T may become significant. When these time delays become somewhat greater than normal human reaction times (i.e. between 0.1 and 1-2 seconds), then the system becomes exponentially more difficult to utilize. Imagine placing a control movement (velocity in this example) to a remote arm while expecting to feel contemporaneous force contact in the manipulator unit. Since there is a time delay T in communication 20 to and from the device (see FIG. 2), the control movement X.sub.m is delayed and the contact force F.sub.s does not return to the user immediately from the remote device 22. As such, the user 26 continues to attempt to move the device forward using a manipulator at the master system 24 intuitively waiting for a sensation of contact response (delayed F.sub.m). Since the remote device response signal F.sub.s is delayed by one second or more, the device has had one or more seconds to move further than anticipated. As such, the device has already contacted and is now pushing very hard on the object before the user receives any force feedback. The user will be pushed back hard by the master manipulator when receiving the very high force feedback which has been delayed by a second or more. This hard motion of the manipulator then propagates through the system with appropriate delays and the system overcompensates by moving away from the object. The process continues as the system quickly oscillates out of control and becomes unusable over a long distance. See, for example, the simulation graph of FIG. 3.
In the past, the problem of such instability has been solved for long distance time-delayed closed loop systems by means of providing damping at one or both of the operator device ends of transmission. As such, a control movement transmitted by the user causes a slow response in the device that tends to cancel out the effect of time delay. Similarly, the force reflected by the device is received by the operator slowly again cancelling out the effects of the time-delayed operation. In particular, a slow force reflection makes the operator feel like less force is present than actually felt by the device and, thus, the system is less prone to overcompensate.
The problem with such extreme damping is that it still cannot guarantee stability in all systems and at all times. Rather, in particular cases, such as when a system phase shift is greater than 180 degrees, the system may remain unstable and incapable of performing useful work.
Alternatively, systems in which response could be predicted were provided with force prediction feedback systems to provide the user with an immediate response sensation at his manipulator. The problem with such a system is that it required certain knowledge of the environment. This limited versatility of the system. Additionally, if prediction assumptions were, in fact, incorrect the system would prove highly unstable.